Vanadyl ion-VO2+ (3 d1) is a probe used in protein studies. EPR is a good method to investigate the binding between VO2+ and protein. Vanadyl-derivative of copper-, zinc-superoxide dismutase were prepared. ESR was used to study the zinc-only (i.e. copper-depleted) superoxide dismutase derivative and the apo enzyme in which both copper and zinc are removed from the protein and native enzyme.